Mergers between clusters of galaxies are the most energetic events in the Universe. Shock waves are generated in the intracluster medium (ICM), galaxies are flung around and reassembled. Some spectacular examples are the "Train Wreck" cluster, the Bullet Cluster, and MACS J0717.

Besides these spectacular mergers, clusters of galaxies grow by accreting smaller galaxies and groups of galaxies. The figure on the right shows the assembly of a galaxy cluster in a cosmological simulation: the bottom right panel is the cluster's end state at redshift z = 0; the other panels (left to right, top to bottom) show the infall of the particles within the cluster from z = 0.877 onwards. There is at least one merger with massive group, and many with smaller galaxies.

As clusters grow by accumulating and assembling smaller groups and individual galaxies, their galaxies are transformed by a variety of physical processes. This 'transformation' of galaxies is necessary to eventually remove gas and slow the rate of star formation. Some transformation processes that remove gas are described here, others include galaxy collisions and removal of material by tidal forces.

These mechanisms operate in smaller groups of galaxies as well as in clusters of galaxies. When galaxies are transformed in smaller groups before being accreted into clusters, they are 'pre-processed'. Additionally, when groups of galaxies are accreted by clusters, the merger process itself can accelerate galaxy transformation: the merger shock results in increased ram pressure and rapid gas removal; the passage of the group core through the cluster core results in increased galaxy-interaction rates. We called this 'post-processing'. Using cosmological and controlled group-cluster simulations, these processes are quantified in Vijayaraghavan & Ricker (2013).

Increased galaxy interaction rates during a merger, due to a short lived increase in galaxy interaction rates.

In addition to impacting when and where the galaxies are transformed, cluster-group mergers and cluster-cluster mergers dynamically reassemble the orbits of their galaxies. This animation shows a cluster merger from two observer perspectives: perpendicular to the direction of the merger (left), and down the direction of the merger (right).

Phase space of a cluster (in blue-green) and an infalling group (in red-orange). The group's core and outskirts evolve independently.

Galaxies that fall in as part of the merging group, and are in the outskirts of the group, 'remember' their infall velocities for many orbits. In real clusters, this means that dwarf galaxies -- the most numerous type of galaxies in clusters -- can be used as tracers of their clusters' infall and formation histories, using their phase space (radial velocity - cluster-centric distance) properties. More details here in Vijayaraghavan, Gallagher, and Ricker (2015).

Can we view these merger remnants in real clusters? Possibly -- watch this space!